A number of systems and techniques are employed to receive and measure various characteristics of a device under test (DUT) using periodically modulated radio frequency (RF) signals output by the DUT, typically in response to an excitation signal. However, it is difficult to perform accurate and complete measurements of broadband periodically modulated RF signals using a conventional mixer-based receiver, as the difference between the minimum and maximum frequencies of the periodically modulated RF signal (referred to as “total bandwidth”) exceeds the intermediate frequency (IF) bandwidth of the receiver (referred to as “IF bandwidth”). The IF bandwidth of a receiver is the bandwidth over which a vector network analyzer (VNA), for example, is able to measure both amplitude and phase of the periodically modulated RF signal.
For example, because of spectral regrowth, the total bandwidth of a broadband periodically modulated RF signal may easily exceed 300 MHz, whereas a standard IF bandwidth of a conventional VNA is only about 40 MHz. Because of the limited IF bandwidth, at least eight local oscillator (LO) frequency settings are needed (e.g., for receiving the 300 MHz total bandwidth in eight 40 MHz bandwidth portions), along with eight corresponding analog-to-digital converter (ADC) data record acquisitions, in order to capture the spectrum between the lowest and highest frequencies of the total bandwidth.
Use of periodically modulated RF signals, which include multiple tones, are commonly used for characterization and validation of various types of DUTs, such as power amplifiers. These signals may be created by repeating IQ-sequences on vector signal generator, for example. However, when the periodically modulated RF signals are mixed with local oscillator (LO) frequencies in a conventional receiver to frequency convert to corresponding IF frequencies, the mixing products of the resulting IF signals include the actual tones at the same tone spacings as the periodically modulated RF signals, as well as unwanted images, such as harmonics and alias components. Images particularly result from high-side mixing, where the LO frequency is higher than the carrier frequency of the periodic modulated signal (by the IF bandwidth). The images create interference and otherwise add noise to the IF signal. Conventional super-heterodyne receivers include specific filters for the purpose of image rejection. However, VNAs and other types of receiver systems, such as Performance Network Analyzers (PNAs), available from Keysight Technologies, Inc., and network analyzers (NAs) do not include such filters. Further, the problem with mixing products generating images is exacerbated when broadband periodically modulated RF signals are involved since multiple frequency conversions using multiple LO frequencies are performed.